This invention relates to a quadrupol ion trap mass spectrometer for molecules.
In prior art quadrupol ion trap mass spectrometers, molecules are injected into an evacuated receptacle in which a ring electrode and two endcap electrodes are configurated and operated in such a manner that molecules with masses larger than a particular cut off mass are trapped. The configuration for generating the required dynamic three dimensional electric quadrupol field has first been described by Paul et al in U.S. Pat. No. 2,939,952. When increasing the RF amplitude on the ring electrode, the lower cut mass increases, and molecules with increasing mass are ejected from the quadrupol ion trap mass spectrometer, and after acceleration, detected with a molecule detector. This mass scanning mode of a quadrupol ion trap and its application for mass spectrometry has been described by Stafford et al in U.S. Pat. No. 4,450,884, Johnson et al in U.S. Pat. No. 5,075,547 and Franzen et al. in U.S. Pat. No. 5,170,054. Prior art quadrupol ion trap mass spectrometers are now commercially available from ThermoFinnigan (San Jose, USA) and Bruker Daltonics (Hamburg, Germany). The principles of a quadrupol ion trap mass spectrometer and its applications are described in reference [Mar00]. Schlunegger et al. have demonstrated that large molecules with masses beyond 100 kDa can be analyzed with quadrupol ion trap mass spectrometers [Sch99].
In prior art quadrupol ion trap mass spectrometers the molecule detector determining the time of ejection of the molecule is an ionizing detector. The disadvantage of ionizing detectors is the strong decrease of detection efficiency for molecules with decreasing velocity [Lin61]. Molecules ejected from a quadrupol ion trap mass spectrometer are accelerated prior to hitting the detector. All molecules have the same acceleration energy and hence molecules with increasing masses have a decreasing velocity. Prior art quadrupol ion trap mass spectrometer using ionizing detectors therefore have a decreasing sensitivity for increasing molecule mass.
It is the objective of this invention to improve the sensitivity of quadrupol ion trap mass spectrometers for massive molecules by using cryogenic particle detectors as molecule detectors.
Cryogenic particle detectors as detectors for molecules in mass spectrometers are described by Twerenbold in U.S. Pat. No. 5,640,010 and a particular embodiment of a cryogenic particle detector for mass spectrometry is described by Frank et al in U.S. Pat. No. 5,994,694. Cryogenic particle detectors have been successfully used as molecule detectors in matrix assisted laser desorption/ionization time-of-flight mass spectrometers (see references [Twe96b], [Twe96c], [Fra96], [Ben 97], [Hil98], [Twe01] and [Twe02]). The mass independent detection efficiency of cryogenic particle detectors and the increased detection sensitivity for massive molecules has been demonstrated [Twe01].
This invention is a quadrupol ion trap mass spectrometer comprising an apparatus for volatilizing and charging molecules out of a mixture; an electro-optical injection apparatus in which the volatilized and charge molecules are accelerated and injected into an evacuated quadrupole ion trap receptacle; the evacuated quadrupole ion trap receptacle containing a ring electrode, an upper end cap electrode and a lower end cap electrode; the three electrodes placed such in the evacuated quadrupole ion trap receptacle that they create an electric quadrupole field in the central region of the evacuated quadrupole ion trap receptacle; the upper end cap electrode having a hole for permitting the entry of the accelerated molecules produced in the volatilizing and charging apparatus and injected from the electro-optical injection apparatus; the lower end cap electrode having a hole for permitting the exit of molecules; a radio frequency alternating current voltage supply connected to the ring electrode operated such that the injected molecules are trapped in the evacuated quadrupole ion trap receptacle; a buffer gas introduced into the evacuated quadrupole ion trap receptacle for cooling the trapped molecules; increasing the amplitude of the radio frequency alternating current voltage on the ring electrode such that the trapped molecules are ejected out of the exit hole of the lower end cap electrode in order of increasing mass-to-charge ratio; an apparatus for accelerating and focusing the ejected molecules; a cryogenic particle detector comprising at least one absorber and at least one sensor for determining the time of impact of the molecules ejected from the exit hole of the lower end cap electrode.
Accordingly, it is an object of this invention to provide a novel quadrupol ion trap mass spectrometer for massive molecules with an increased sensitivity for large mass molecules.
It is a still further object of the invention to provide a novel apparatus for measuring the masses of molecules in the single particle counting mode providing the additional information of the energy of the individual molecule.
It is a still further object of the invention to provide a novel apparatus for measuring the charge of the individual molecules.
Those objects are achieved by using cryogenic particle detectors in a quadrupol ion trap mass spectrometer. With a cryogenic particle detector, the absorbed kinetic energy of the impacting accelerated molecules is measured, providing both the arrival time and the energy of the molecule ejected from the quadrupol ion trap mass spectrometer. Because the kinetic energy of the molecules ejected from the quadrupol ion trap mass spectrometer, and subsequently accelerated and focused onto the detector, is independent on the mass of the molecule, a cryogenic particle detector yields a mass independent sensitivity.
In accordance with the above and further objectives of the invention, one embodiment of apparatus is a quadrupol ion trap mass spectrometer with a superconducting tunneling junction as the cryogenic particle detector.
In another embodiment, a quadrupol ion trap mass spectrometer is equipped with a microcalorimeter as the cryogenic particle detector.
In another embodiment, a quadrupol ion trap mass spectrometer is equipped with a superconducting strip detector as the cryogenic particle detector.
From the above summary, it can be understood that the quadrupol ion trap mass spectrometer of this invention has several advantages: (1) it allows the mass determination of a molecule with a sensitivity independent of the mass of the molecule, i.e. there is no decrease in sensitivity for massive molecules; (2) it allows the determination of the energy of an individual molecule ejected from the quadrupol ion trap mass spectrometer; (3) the charged state of this molecule can be determined directly by measuring the pulse height of the cryodetector signal; (4) it allows the reduction of background events of fragmented molecules following the acceleration after ejection from the quadrupol ion trap mass spectrometer.